The invention relates in general to electrical switches, on load tap changers, circuit breakers, reclosers, and more particularly to electrical contacts and electrical switches utilizing the same.
Electrical switches that operate while under load (with current flowing) are susceptible to certain limits at which further use will result in equipment failure. For example, components that overheat during normal equipment operation will, at some point, reach a limit at which they must be replaced. This condition can also have catastrophic consequences and has the potential for failure of valuable infrastructure assets and loss of life. The overheating of the electrical contacts causes failure of switches or deteriorated switch operation and otherwise generally reduces or limits the useful lives of the switches themselves. The degree of deterioration from overheating is a function of the various conditions that exist during operation, such as the amount of current carried by the contacts, the voltage applied across the contacts, the maximum temperature experienced, along with the severity of service under which the contacts operate. In addition, overheating of electrical contacts can signal failure or malfunction of other switch components. Switches are also subject to overheating from a high resistive contact interface. Excessive heating of contacts or other switch components can dramatically change the electrical and mechanical characteristics of the contacts and the ability of the switch to properly operate. Further, it can cause carbon accumulation (coking), and failure of the switch through an inability to operate or a type of failure known as a “flash-over”.
As a result of the consequence described, utility companies spend hundreds of thousands of dollars annually and commit a considerable amount of human resources to monitor their high voltage electrical equipment for signs of abnormal conditions that indicate overheating is occurring and failure is possible or imminent.
There are four basic environments within which electrical contacts operate: (1) air; (2) inert gas; (3) oil; and (4) within a vacuum. Electrical contacts are used for low, medium and high voltage equipment, including circuit breakers, transformer and regulator load tap changers, and reclosers. These contacts operate under oil, under pressurized gas (e.g., SF6), in an enclosure open to ambient air, or under vacuum. Electrical contacts that operate under oil or gas do so within a containment vessel or compartment, preventing easy access to the contacts. As such, regardless of the type of environment in which contacts and other components operate, they operate within some form of enclosure. Each of these environments presents challenges to the contact monitoring process.
Because overheating of electrical contacts cannot be eliminated, users of switches must monitor the switch to detect when the switch experiences overheating to a predetermined critical point as prescribed by the utility or end user. Monitoring of the switch for overheating includes: Sampling the surrounding oil, sampling the gasses in the headspace above the oil, or sampling the primary gas and performing dissolved gas analysis (DGA) through the use of gas chromatography; the use of Infrared scanning of the external surfaces of the switch containment vessel or compartment, and; the use of external temperature monitors to detect the temperature of the containment vessel or compartment, and internal temperature sensors that measure the temperature of the oil.
A transformer has two sets of wire coils, known as the primary windings and the secondary windings. A voltage applied to the primary windings (also referred to herein as the “primary voltage”) will induce a voltage in the secondary windings (also referred to herein as the “secondary voltage”). The secondary voltage is typically higher or lower than the primary voltage, depending upon the relationship of the number of turns, or coils, and of wire in the primary and secondary windings of the transformer. A transformer with a greater number of coils in the secondary windings will produce a secondary voltage higher than the primary voltage. A transformer without taps, or access points, in the secondary windings will produce only one secondary voltage for each primary voltage. Many examples of transformers have numerous taps in the secondary windings so a variety of secondary voltages may be selected from one transformer. A transformer which has taps in the secondary windings will allow several secondary voltages to be accessed, depending upon which tap is selected. One transformer may be used to both decrease and increase voltage, if it is tapped at points lower and higher in number than the number of turns in the primary windings. A “coil tap selector switch” or a “load tap changer” must be provided, however, to switch between the various secondary winding taps.
A “load tap changer” is a mechanical device that moves a moving electrical contact to different stationary tap contacts within the switch, depending on the voltage output required. Current practices, however, include the application of advanced diagnostic tools and have resulted in extending the maintenance interval with little or no regard to the number of operations.
Some of the methods used previously to monitor electrical equipment performance which attempted to overcome the effort and expense required by direct physical inspection include the following:
1. Dissolved Gas Analysis (DGA)
Dissolved gas analysis is used for monitoring the condition of electrical contacts that operate in an oil environment. The method includes extracting a sample of the oil surrounding the contacts and analyzing it using gas chromatography to determine the amounts and correlation of key gasses generated during operation. The resulting values, collectively, is indicative of various types of problems that may be occurring within the equipment. For example, the presence of acetylene dissolved in the oil is indicative of arcing, and its correlation to ethylene is a key consideration for detecting overheating and coking. This process, however, lacks the precision necessary to determine the point at which overheating reaches the temperature at which failure is possible or imminent as the tests are performed intermittently and failures continue to occur as a result.
2. Infrared Monitoring
Infrared monitoring may be used in an air, inert gas, vacuum, or oil environment. The method includes the use of an infrared camera to monitor the external temperature of high voltage equipment. Temperature and resistance are directly related. As resistance to current flow through electrical equipment increases, the temperature of the oil also increases. The infrared camera measures in a general sense the temperature increases and alerts the user accordingly. However, this system is inexact because it cannot monitor the temperature of contacts or other components separately from other neighboring components within the enclosure. As a result, the utility does not know what components will require replacement when the switch is opened for repair.
3. Temperature Differential Monitoring
Temperature Differential Monitoring consist of temperature sensors applied directly to the outside surfaces of both the switch compartment and the outside of the main transformer tank. Temperature sensors attach to instrumentation that measures and logs the temperature in real time. Most utility companies schedule internal inspection when the temperature differential between the switch compartment and the main transformer tank reaches 10° C.
The above diagnostic methods have proven to be useful in a general sense for identifying overheating and coking. These methods, however, do not have the ability to distinguish when the contacts have overheated to their limit of service life or that failure of the switch is possible or imminent. In addition, typical sampling intervals also present the possibility that oil analysis could not detect an upset condition prior to failure. Peak efficiency can only be achieved where a method exists that provides detection of overheating of electrical contacts when they have reached a prescribed temperature.
Accordingly, there exists in the industry a need to provide a temperature indicator for electrical contacts that will provide a means of detecting overheating and provide an indication to users that a certain critical temperature has been reached.